Organic carboxylic acids and their esters constitute important starting materials for chemical synthesis. It is of particular economic significance to prepare such organic carboxylic acids and their esters at minimum cost. The fermentative preparation of carboxylic acids using suitable microorganisms is gaining increasing importance. However, a disadvantage in these processes is that the carboxylic acids are usually not obtained as the pure substance and additionally not in the form of the free acid, but rather neutralized partly or fully in salt form. Neutralization is usually necessary, since the fermentation broth would otherwise overacidify with increasing fermentation time and, as a consequence thereof, the microorganism would be damaged. The pure preparation of carboxylic acids or the corresponding esters synthesized fermentatively but also chemically is therefore of great significance.
Starting from carboxylic acids prepared fermentatively, especially lactic acid or succinic acid, a wide variety of different products of value are obtainable by a chemical route with the aid of different synthesis strategies (cf., for example, review article by Varadarajan et al., Biotechnol. Proc. 1999, 15, 845-854).
The prior art also describes various strategies for working up carboxylic acids, especially for esterifying carboxylic acids. For example, WO-A-00/64850 addresses the preparation of organic acids from the corresponding ammonium salts by thermal decomposition of an aqueous ammonium salt solution of the organic acid in the presence of an alcohol and removal of a vaporous mixture of ammonia, water and alcohol from the reaction mixture. If desired, the organic acid formed can be converted to an ester. However, the maximum ester yield in this process is obviously very low; for instance, in the working examples there, a maximum yield of only 14.6% is described for ethyl lactate. This process thus appears to be unsuitable for the quantitative conversion of an ammonium salt of an organic acid to the corresponding ester. Furthermore, the usability of the system described there for esterifying polycarboxylic acids is not confirmed by any examples whatsoever. Moreover, the process described there possesses the disadvantage that reaction products with an undesirably high proportion of incorporated nitrogen are obtained. For instance, a content of incorporated nitrogen (lactamide content) of 16.7 mmol is calculated from the nitrogen analyses described therein (example 8 therein) for the product-containing bottoms in spite of two repetitions of the stripping, which, when converted, corresponds to about 18.9% of the nitrogen used (approx. 88 mmol of ammonium lactate).
The esterification of carboxylic salts prepared by fermentation by catalytic reaction of a salt/alcohol mixture and subsequent removal of the ammonia and water formed by pervaporation is described in WO-A-98/23579. However, a disadvantage in this process is the necessity of the use of pervaporation membranes. Furthermore, in the working examples there, esterification rates of not more than 40.7% (in the conversion of ammonium propionate to the corresponding ethyl ester) are described. This process too thus appears to be unsuitable for a highly quantitative esterification of ammonium salts of carboxylic acids. Furthermore, the usability of the system described there for esterifying polycarboxylic acids is not confirmed by any examples whatsoever.
The single-stage or multistage esterification of free mono-, di- or polycarboxylic acids by reacting a carboxylic acid/alcohol mixture in a reactor is described in WO-A-20051051885. However, there is no indication of the usability of this process for the esterification of corresponding ammonium salts.
The provision of ester-containing fuels and chemicals from biomass by reacting biologically prepared alcohols, especially methanol and ethanol, and carboxylic acids prepared by fermentation, especially their ammonium salts, is proposed by Olson et al. in Applied Biochemistry and Biotechnology, 2003, Vol. 105-108, 843-851. Various methods for preparing monocarboxylic esters are discussed. The successful esterification of polycarboxylic salts is not described in this document.
DE-A-198 29 809 describes the esterification of a mixture of water and free monocarboxylic acid by reactive distillation.
In addition, none of the documents cited addresses the controlled avoidance of undesired nitrogen-containing by-products of the esterification reaction, especially of chemically stable carboximides, whose formation in the esterification of ammonium carboxylates constitutes a significant problem. In addition, such imides can be removed from the reaction mixture only with difficulty and can have a disruptive effect in later chemical reactions.
It is therefore an object of the present invention to provide an improved process for preparing polycarboxylic esters from the corresponding ammonium salts, which firstly ensure a high ester yield and secondly largely prevent the formation of undesired nitrogen-containing by-products, especially of carboximides.